Page 385 - Handbook of Energy Engineering Calculations

have very high capital costs.
In the tropics, the ocean surface temperature often exceeds 25°C (77°F).
One kilometer below the surface the water temperature is usually no higher
than 10°C (50°F). The concept of OTEC is based on the utilization of this
temperature difference in a heat engine to generate power, a concept first

recognized by the Frenchman d’Arsonval in 1881.
The maximum possible efficiency of a heat engine operating between two
temperature limits cannot exceed that of a Carnot cycle operating between the

same temperature limits. Because of the temperature drops in steam or other
vapor generators, and the condenser in an actual system, inefficiencies in the
turbine and pumps, and other inefficiencies, the efficiency of a real OTEC
power plant seldom exceeds 2 percent.
The extremely low efficiency of an OTEC system implies extremely large

power plant heat exchangers and components. At 2 percent efficiency the
heat exchangers must handle 50 times the net output of the plant. Although
there are no fuel costs, the capital costs are extremely high, as are the unit

capital costs, $/kW. In addition to the large size per unit of power generation,
the developmental problems and the uncertainties of market penetration make
the financial risks associated with the development of large OTEC
technologies so high as to effectively preclude most utilities. The first
Calculation Procedure is this section demonstrates OTEC efficiency

computation.
Ocean waves have long been looked on as a source of power. Like wind
power, and OTEC power, ocean and sea waves are caused indirectly by solar

energy. Waves are caused by the wind, which in turn is caused by the uneven
solar heating and subsequent cooling of the Earth’s crust and the rotation of
the Earth. Wave energy, at its most active, however, can (like wind energy)
be much more concentrated than incident solar energy, even at the latter’s
peak. Devices that convert energy from waves can therefore produce much

higher power densities than solar devices.
The total energy of a wave is the sum of its potential and kinetic energies.
Wave energy conversion is done by floats, accumulator machines, dolphin-

type generators, dam-atoll conversion devices, multi-pontoon rafts, and
others. An accompanying procedure shows a typical computation for the
energy and power densities of waves.
The tides are yet another source of energy from the oceans. This energy